The ability to detect misfiring cylinders (specifically, dead cylinders) in an internal combustion engine has been mandated by the California Air Resources Board (CARB) as part of the On Board Diagnostics (OBD II) program targeted primarily for diesel engine-based vehicles. Under OBD II, each cylinder must be monitored at least once per driving cycle and any misfiring cylinder, as well as whether multiple cylinders are misfiring, must be identified. Upon detection of a sufficient number of misfires, OBD II requires that the driver be notified of such a fault condition via illumination of a driver-observable lamp (MIL). Manufacturers of vehicles having diesel engines must comply with OBD II by Jan. 1, 1998 for any such vehicles sold thereafter in California.
Various prior ark devices have been employed to sense, and possibly thereafter act upon, engine misfire. One such misfire detection system is disclosed in U.S. Pat. No. 5,213,081 to Fujimoto. Fujimoto discloses a system wherein engine speed at various crank angles is detected and thereafter compared with corresponding engine speeds at a subsequent firing cycle. If a particular cylinder is detected as possibly misfiring, fuel to that cylinder is decreased to a level sufficient to induce misfiring and the engine speed corresponding to the crank angle for that cylinder is compared to its previous value. If the two speeds are sufficiently similar, then a misfire is verified.
Another misfire detection system is disclosed in U.S. Pat. No. 4,366,793 to Coles, wherein engine speeds for each cylinder are compared to their corresponding speeds at the previous firing cycle, and misfires are detected if the difference in any of such speeds is below a reference threshold. If a misfire is so detected, the fuel-to-air mixture for that cylinder is increased in an attempt to compensate for the misfire due to an excessively lean fuel mixture.
Several other U.S. Patents including U.S. Pat. Nos. 5,303,158 to Kuroda, No. 5,088,318 to Osawa, No. 4,932,379 to Tang et al., No. 4,930,479 to Osawa et al. and No. 4,928,228 to Fujimoto et al., disclose similar misfire detection systems wherein, under normal fueling conditions, each cylinder's contribution to engine speed is monitored and compared to its contribution at the previous firing cycle. Such speed differences are thereafter used, according to various approaches, to detect a misfire condition.
What all these efforts have in common is that they are "passive" monitors; that is, they monitor each cylinder's contribution to engine speed under normal fueling conditions and register a misfire upon detection of a characteristic deceleration. Such systems are susceptible to incorrectly detecting engine misfire, i.e. false positive failures, owing to calibration errors, fueling inconsistencies and lag times and other fuel system control problems. Moreover, "weak" misfire conditions that may not be readily discernable at low idle conditions may not be detected in such systems. What is therefore needed is a misfire detection system that actively challenges an engine's response to changing fueling conditions. Such an "active" misfire detection strategy should be inherently more robust than a "passive" misfire detection strategy, and thereby generally lead to fewer incorrectly detected engine misfires.